Fix #794: Avoid error when superuser views org member edit page

[seav]

Proposed changes in this pull request

• Fix Superuser cannot view org member edit page #794:
  • Add exception handling to OrganizationMembersEdit.get_context_data() to avoid the error when the user is a superuser.
  • Add a unit test to catch this bug and to verify that it is fixed.
• Disable the org role select form field when the viewing user is an org admin viewing his/her own member page.
  • (Context: 1aac076#commitcomment-19327831)
  • Update a functional test to reflect the disabled select state.

When should this PR be merged

Anytime.

Risks

None foreseen.

Follow up actions

None.